Various methods to metallise substrates are known in the art.
Conductive substrates can be directly plated with another metal by various wet chemical plating processes, e.g. electroplating or electroless plating. Such methods are well established in the art. Usually, a cleaning pretreatment is applied to the substrate surface before the wet chemical plating process is applied to ensure a reliable plating result.
Various methods are known of coating non-conductive surfaces. In wet chemical methods, the surfaces to be metallised are, after an appropriate preliminary treatment, firstly catalysed and then metallised in an electroless manner and thereafter, if necessary, metallised electrolytically. With the introduction of more advanced technologies, hitherto used organic substrates are less suitable because of their relatively poor dimensional stability and coplanarity, which limits them in terms of Input/Output (I/O) pitch. Inorganic interposers made of silicon or glass allow for straightforward matching of the Coefficient of Thermal Expansion of the interposer to the Silicon Chip. Silicon has a mature manufacturing base but suffers from some disadvantages when compared to glass. In particular glass has inherently superior electrical properties than silicon and offers the possibility to use larger area panel sizes, which results in significant cost savings versus a wafer based platform. A reliable plating technology for good adhesion of copper to glass is a critical prerequisite for the use of glass substrates in the electronic packaging market.
This is a challenge however, as metallization of a very smooth glass with a surface roughness of <10 nm is significantly more challenging than plating on an organic substrate. Methods that depend solely on mechanical anchoring from substrate roughening were tested for adhesion performance.
However, this requires strong roughening of the substrate surface which negatively impacts the functionality of the metallised surface, e.g. in printed electronic circuits or Radio Frequency Identification (RFID) antennas.
Wet-chemically etching with either HF containing acidic media or hot NaOH, KOH or LiOH containing alkaline media can be employed for both cleaning and roughening of the non-conductive substrates, particularly glass or ceramic type substrates. Adhesion is then provided by additional anchoring sites of the roughened surface.
In EP 0 616 053 A1 there is disclosed a method for direct metallisation of non-conductive surfaces, in which the surfaces are firstly treated with a cleaner/conditioner solution, thereafter with an activator solution, for example a colloidal palladium solution, stabilised with tin compounds, and are then treated with a solution which contains compounds of a metal which is more noble than tin, as well as an alkali hydroxide and a complex former. Thereafter the surfaces can be treated in a solution containing a reducing agent, and can finally be electrolytically metallised.
WO 96/29452 concerns a process for the selective or partial electrolytic metallisation of surfaces of substrates made from electrically non-conducting materials which for the purpose of the coating process are secured to plastic-coated holding elements. The proposed process involves the following steps: a) preliminary treatment of the surfaces with an etching solution containing chromium (VI) oxide; followed immediately by b) treatment of the surfaces with a colloidal acidic solution of palladium-/tin compounds, care being taken to prevent prior contact with adsorption-promoting solutions; c) treatment of the surfaces with a solution containing a soluble metal compound capable of being reduced by tin (II) compounds, an alkali or alkaline earth metal hydroxide, and a complex forming agent for the metal in a quantity sufficient at least to prevent precipitation of metal hydroxides; d) treatment of the surfaces with an electrolytic metallisation solution.
Alternatively, conductive polymers can be formed on the non-conductive surface to provide a first conductive layer for subsequent metal plating of the surface.
US 2004/0112755 A1 describes direct electrolytic metallisation of electrically non-conducting substrate surfaces comprising bringing the substrate surfaces into contact with a water-soluble polymer, e.g. a thiophene; treating the substrate surfaces with a permanganate solution; treating the substrate surfaces with an acidic aqueous solution or an acidic microemulsion of an aqueous base containing at least one thiophene compound and at least one alkane sulfonic acid selected from the group comprising methane sulfonic acid, ethane sulfonic acid and ethane disulfonic acid; electrolytically metallizing the substrate surfaces.
U.S. Pat. No. 5,693,209 is directed to a process for directly metallizing a circuit board having non-conductor surfaces, includes reacting the non-conductor surface with an alkaline permanganate solution to form manganese dioxide chemically adsorbed on the non-conductor surface; forming an aqueous solution of a weak acid and of pyrrole or a pyrrole derivative and soluble oligomers thereof; contacting the aqueous solution containing the pyrrole monomer and its oligomers with the non-conductor surface having the manganese dioxide adsorbed chemically thereon to deposit an adherent, electrically conducting, insoluble polymer product on the non-conductor surface; and directly electrodepositing metal on the non-conductor surface having the insoluble adherent polymer product formed thereon. The oligomers are advantageously formed in aqueous solution containing 0.1 to 200 g/l of the pyrrole monomer at a temperature between room temperature and the freezing point of the solution.
Ren-De Sun et al. (Journal of the Electrochemical Society, 1999, 146:2117-2122) teach the deposition of thin ZnO layers on glass by spray pyrolysis, followed by wet chemical Pd activation and electroless deposition of Cu. They reported a moderate adhesion between the deposited copper layer and the glass substrate. The thickness of the deposited copper is about 2 μm.
EP 2 602 357 A1 relates to a method for metallization of substrates providing a high adhesion of the deposited metal to the substrate material and thereby forming a durable bond. The method applies novel adhesion promoting agents comprising nanometer-sized oxide particles prior to metallization. The particles are selected from one or more of silica, alumina, titania, zirconia, tin oxide and zinc oxide particles which have at least one attachment group bearing a functional chemical group suitable for binding to the substrate. The particles are functionalized by having at least one attachment group bearing a functional chemical group suitable for binding to the substrates. These nanometer-sized particles are attached to the substrate and remain chemically unchanged before a subsequent metal plated layer is attached to the substrate surface.
JP H05-331660 A relates to the formation of a copper oxide film on a substrate bearing a zinc oxide layer. Such method comprises the following steps i) apply a zinc acetate solution to a substrate surface, ii) deposit a copper layer onto the substrate surface, iii) oxidise the plated copper layer to form a copper oxide at a temperature of about 300 to 500° C. in an oxygen containing atmosphere, iv) partially reduce the copper surface and v) form an electrolytic copper coating. The method is not directed to formation of metal oxide layers for adhesion promotion. Depending on the chemical nature of substrate surface, the type of the plated metal and the thickness of the plated metal layer, adhesion of the plated metal layer to said surface can be an issue. For example, adhesion can be too low to provide a reliable bond between the metal layer and the underlying substrate.
Furthermore, such methods tend to require additional steps in the substrate preparation that are not typically easily controllable for uniform surface roughness.
Moreover, the problem of large CTE (coefficient of thermal expansion) mismatch between glass (CTE=3-8 ppm) and subsequently plated metal, typically copper (CTE=about 16 ppm) is not addressed, which often leads to delamination from the bare glass.